1. Field of the Invention
The present invention is concerned with a lamp drive circuit that periodically energizes a lamp above its rated voltage and current for rapid illumination of the lamp. More particularly, the invention is concerned with a circuit in which the lamp is connected in a voltage divider and the circuit monitors the voltage drop across the lamp. The voltage drop is indicative of lamp resistance and thereby temperature. When the voltage drop reaches a predetermined limit corresponding to the temperature rating of the lamp, it is de-energized and allowed to cool during the remainder of the cycle thereby enabling a high modulation frequency.
2. Description of the Prior Art
Some measuring instruments measure the absorption or transmittance of certain light wavelenghts from a light source as a measure of a parameter of interest such as carbon dioxide. In order to eliminate the effects of ambient light and other noise components, the light from the light source is modulated, which allows filtering of DC inaccuracies. Such modulation is typically accomplished by using a mechanical, rotating filter wheel. Such mechanisms are relatively expensive, bulky and prone to mechanical failure.
Electronic modulation of the light source itself would eliminate the need for mechanical modulation, but available solid state devices such as LEDs and laser diodes cannot produce some needed frequencies such as the 4.2 uMeter wavelength necessary for carbon dioxide detection. A standard tungsten filament lamp can provide needed wavelengths but cannot be modulated electronically with a high enough modulation frequency because of the long thermal time constant of the filament.